In drilling wells, a hollow tubular drill string is introduced into the well. Adjacent the bottom of the drill string, a drill tool casing houses a drive motor which rotates a central shaft which projects beyond the drill tool casing and mounts a drill bit. Rotation of the drill bit extends the length of the well as the drill tool casing is advanced incrementally.
The drive motor for the drill shaft may be hydraulic or pneumatic, of the positive displacement (PDM)or turbine type mounted within the drill tool casing. Conventionally, the motor is driven by forcing air or drilling mud through the casing and the motor. Drilling mud may consist of clay, water and/or oil, weighting material such as barium sulfate or hematite, sand, quartz, various types of pulverized, granulated or chips of abrasive material, and chemical polymers. Most frequently, drilling mud is pumped from the surface and through the drill string into the drill tool casing so that it fills the interior of the drill tool casing and the well annulus. The mud is formulated such that it exerts an isostatic pressure which increases on the order of 0.5-1.0 pounds per square inch per foot of depth in the well so that the isostatic pressure may be comparable to subterranean pressure at the bottom of the well in order to prevent well collapse Thus, at the bottom of a 15,000-foot well, the pressure in the drilling mud may be 15,000 psi.
The mud within the drill tool casing is used to drive the fluid motor, and the mud exhausted from the motor is directed through nozzles in the drill bit so as to expel chips and other material disintegrated by the drill bit and also to provide a coolant for cooling the cutting surfaces of the drill bit to improve its cutting efficiency. The pump is normally at the surface adjacent the well, and supplies mud at a pressure sufficient to overcome the pressure drop due to friction of the mud flowing through the drill string. This pressure is further elevated above isostatic pressure so as to drive the motor when its outlet is at isostatic pressure plus the pressure required to exhaust the mud through the nozzles in the drill bit, and upwardly through the annular space around the drill tool casing and along the entire length of the drill string to the surface for filtration and recycling.
In recycling the drilling mud, the larger particles of drilling debris are filtered from the mud in filters and settling tanks, but the constituents of the drilling mud themselves may still have high abrasive character. Although it is possible to design the fluid motor to withstand the flow of abrasive drilling mud through the motor, the bearing devices which center the rotary shaft within the drill tool casing are simultaneously subject to large fluctuating axial and radial mechanical forces and severe abrasion by the drilling mud. Conventional long-life sealed lubricated bearings are not available for use in an environment where the pressures may vary from atmospheric to more than 20,000 psi, and where the pressure medium is an abrasive fluid such as drilling mud. Substantial down time and cost is required to withdraw the drilling string from the drill hole for maintenance operations upon the motor, the bearing assembly, and the associated drill bit.